CO2 slab lasers include a pair of rectangular, plane, metal electrodes mounted within a sealed housing containing a laser gas mixture including CO2 and inert gases. The electrodes are parallel to each other and spaced close together to define a slab-shaped discharge region. RF power is used to excite the gas mixture to for generating laser radiation. A description of such a laser can be found in U.S. Pat. No. 5,140,606 assigned to the assignee of the present invention and the complete disclosure of which is hereby incorporated herein by reference.
This type of laser typically includes a hybrid optical resonator. The resonator is an unstable resonator in the width-dimension of the parallel spaced apart electrodes and a waveguide type resonator in a dimension perpendicular to the plane of the electrodes. In early models the unstable resonator was a positive branch unstable resonator. In later models a negative branch unstable resonator was preferred.
A positive branch unstable resonator designs is about an order of magnitude more difficult to align than a negative branch unstable resonator but is much less sensitive to output beam pointing variations that result from changes in the curvature of mirrors of the resonator, which changes result in turn from changes in the temperature of the mirror. A negative branch unstable resonator is much more sensitive to temperature induced mirror curvature changes. Beam-pointing variations are a problem in most applications where the laser beam must be steered or directed accurately to a particular location or locations on a workpiece.
Analysis indicates pointing-variations of a laser beam scale directly with the width of a negative branch unstable resonator and inversely as the square of its length. Designing a shorter industrial CO2 laser is looked upon with favor in the industrial application of CO2 lasers provided beam quality is not compromised. As the length of the discharge is made shorter, the width of the discharge needs to be increased to maintain the same discharge area required to obtain the same laser output power.
A negative branch unstable resonator includes an output coupling mirror and a return mirror each having concave reflecting surfaces. The surfaces are made highly reflecting by depositing multilayer thin film coatings on the surfaces. The output coupling mirror is shorter than the return mirror in order to provide for an output for the laser beam past the mirror. The return mirror normally extends over the entire width of the discharge generated by the parallel facing electrodes. The output coupling mirror is shorter to allow a portion of the radiation circulating in the resonator to bypass the mirror as output radiation.
When a laser beam is circulating in the resonator the reflecting surfaces of the mirrors are heated as the laser mirrors have a small, but finite, optical absorption. When the laser is suddenly turned on to a sufficiently high full power, rapid heating of the reflective surface causes the surface to suddenly distort. The mirror becomes suddenly less concave, i.e., suddenly has an increased radius of curvature. This sudden increase in the curvature radius causes the laser beam to suddenly point in another direction. The radius of curvature quickly recovers to nearly its original radius as the fast transient heating is quickly conducted away by the mirror body.
The heat from the reflecting surface eventually propagates through the thickness of the mirror body establishing a temperature gradient between the front and back surfaces of the mirror. This thermal gradient further cause the mirror to become less concave until a steady state mirror curvature is reached at a given laser power. The back of the mirror is typically attached to a large metal plate, which is an end flange of a sealed housing in which the resonator and laser gas are enclosed. This causes the back of the mirror to be cooler than the front surface. The difference in the time response between the transient and steady-state mirror radius change is over two orders of magnitude.
Under low laser pulse repetition frequency (PRF) operation, the mirror radius changes directly in response to the changes in the PRF. As the PRF increases, the thermal time constant of the mirror assembly begins to average out the time variations in the mirror radius of the mirror. The PRF at which the averaging begins is dependent on the thermal time constant of the material from which the mirror is made and the mass of the mirror.
One arrangement directed at minimizing mirror curvature changes under steady state operation is described in U.S. patent application Ser. No. 12/168,376, filed Jul. 7, 2008 (U.S Pre-Grant Publication No. 20090034577), assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference. This result is achieved by designing a mirror with a mirror body of a particular shape with strips of a material different from that of the body attached to the body to provide a compensating bi-metallic effect. An example of the arrangement is depicted in FIG. 1 and FIG. 1A.
Here, the mirror-arrangement 10 includes a metal mirror-body 12 having a generally T-shaped cross-section, with a head portion 14 and a stem portion 16. A concave surface is generated, polished, and coated on the base of the stem portion to provide a concave reflective surface 18 having a radius of curvature R. Typically, the width L of the reflective surface is about equal to the width of the slab discharge for a turning mirror and somewhat shorter, for example between about 12% and 17% shorter than the discharge width for an output coupling mirror to allow output to be coupled out of the resonator. The height h of the reflective surface is typically about six times the height of the discharge, i.e., six times the separated of the discharge electrodes.
Strips 17 of a metal different from that of body 12 are bolted to the underside of the head-portion of the body. In an example described in the patent publication, the body 12 is made from copper and the strips 17 are made from stainless steel. The purpose of the strips is create a bimetallic stress that in steady-state operation, will compensate for differential expansion of the body that tends to increase the radius of curvature of the mirror due to a front-to-back thermal gradient in the mirror.
Mirror arrangement 10 was designed for use in a slab laser having an average power of about 400 kilowatts (kW). The arrangement was successful in compensating long term curvature changes at that power to an extent described in the above referenced '577 publication.
Subsequently, a mirror having this configuration was used in a laser having an average output power of 1.5 kW (about 4 times the original design power). In this case, a very strong transient change in beam pointing was observed immediately after turning on the laser at the 1.5 kW power.
FIG. 1B is a graph schematically illustrating pointing stability (far-field angular beam-position as a function of time) of a slab laser having a power of about 1.5 kW average, and including mirrors designed according to the arrangement of FIG. 1. Power output was at 60% duty cycle with at a (PRF) of 10 kHz. Output coupling was 12%. It can be seen that immediately after the laser was turned on, there was a beam deflection of 400 microradians (μrad) in about 0.75 seconds with the beam assuming to a more or less constant deflection of about 450 μrad, within about one-second, over the time period of the graph.
In most laser-processing operations a work-piece is positioned in the laser-beam path before the laser is turned on, and the material processing occurs sufficiently quickly that beam pointing uncertainty of even one-second duration is significant and can adversely affect the processing operation. Accordingly, it would be advantageous to minimize if not altogether eliminate, transient pointing variations, however short, of the type exemplified by the graph of FIG. 1B.